Tire Size Chart

The 60-second rule

Sidewall (in) = width (mm) × aspect ratio % ÷ 25.4. Diameter = sidewall × 2 + rim. Circumference = π × diameter. Revs/mile = 63,360 ÷ circumference. For 215/65R16: sidewall = 215 × 0.65 ÷ 25.4 = 5.50 in; diameter = 5.50 × 2 + 16 = 27.00 in; circumference = π × 27.00 = 84.84 in; revs/mile = 63,360 ÷ 84.84 = ~747. The aspect ratio trips people up most: 65 is not 65 mm — it is 65% of the 215 mm width, a percentage, not a measurement.

The worked example, line by line

Setup: decode P215/65R16 — the most common example on the internet and a real compact-SUV size.

P = passenger. The prefix is the service type: P = passenger vehicle. LT = light truck, ST = special trailer, T = temporary spare, C = commercial. No letter = Euro-metric size.

215 = section width in millimeters. Measured sidewall to sidewall across the tire’s widest point: 215 mm (≈ 8.46 in). This is the nominal width, not the tread’s contact patch.

65 = aspect ratio. The sidewall height is 65% of the width: 215 × 0.65 = 139.75 mm = 5.50 in. Lower numbers = shorter, sportier sidewalls; higher = taller, cushier ones.

R = radial. The internal ply cords run radially; virtually every modern passenger tire is radial.

16 = rim diameter in inches. This tire fits a 16-inch wheel — this number must match your wheel exactly.

The totals: diameter = 5.50 × 2 + 16 = 27.00 in; circumference = π × 27.00 = 84.84 in; revs/mile = 63,360 ÷ 84.84 = 747.

Branch — comparing 225/65R17 against it: the new size is 28.52 in tall vs 27.00 in — a +5.6% change. At an indicated 60 mph your real speed is 63.4 mph, and it fails the ±3% rule: too tall, speedometer under-reads, clearance and gearing are off.

The printable reference tables

Common sizes with their computed diameter, circumference, and revolutions per mile, plus the load-index and speed-rating charts. Use your browser’s print function for a hard copy — the tables are print-styled.

Print the chart

Output · formula · meaning

Who uses this

• The used-car buyer checking the door sticker: the tires read 225/65R17 but the door jamb says 215/65R16 — the comparer shows a +5.6% fail and a previous owner who upsized too far.
• The winter-tire shopper: decodes a 205/60R16 winter option (25.69 in) against their summer 215/65R16 and checks whether the change stays within ±3%.
• The student asking “what do the numbers on a tire mean”: types P215/65R16 and gets each segment explained — prefix, width, aspect ratio, radial, rim — with the real dimensions underneath.
• The truck owner eyeing LT tires: decodes LT245/75R16 (30.47 in, 662 revs/mile) and reads the load-index chart to make sure the new rubber carries the truck’s weight.

Accuracy & limitations

Sizes are nominal: the numbers describe an engineered class, not a micrometer reading of your tire. Actual mounted dimensions vary about ±1–2% by manufacturer, model, tread depth, rim width, and inflation — a “27.00 in” tire may measure 26.7–27.3 in on the car. The ±3% rule is the industry guideline for keeping speedometer error, gearing, ABS calibration, and fender clearance acceptable — it is a rule of thumb, not a law, and your vehicle maker’s recommendation beats it. Load index and speed rating: always meet or exceed the door-jamb sticker values, not the sidewall’s — the sticker is your vehicle’s requirement; the sidewall is the tire’s capability. Check the door jamb first: the tires currently mounted may not be the factory size, so decode the sticker before shopping.